Many different digital systems, such as GSM, DCS, PCS, D-AMPS, are used for cellular networks, frequently within the same service area. These systems typically operate in different frequency ranges--for example, the GSM system operates at 890-960 MHZ, and the DCS system operates at 1710-1880 MHZ. In service areas where multiple frequency ranges are used, a dual mode mobile telephone or mobile data terminal which can operate in different systems is desirable. Dual mode phones have been developed which require dual band miniature antennas.
Some efforts on the dual band antennas design has been done. The Japanese patent (6-37531) discloses a helix which contains an inner parasitic metal rod. In this antenna, the antenna can be tuned to dual resonant frequencies by adjusting the position of the metal rod. Unfortunately, the band width for this design is too narrow for use in cellular communications. Dual band printed monopole antennas are known in which dual resonance is achieved by the addition of a parasitic strip in close proximity to a printed monopole antenna. While such an antenna has enough bandwidth for cellular communication, it requires the addition of a parasitic strip. Moteco AB in Sweden has designed a coil matching dual band whip antenna and coil antenna, in which dual resonance is achieve by adjusting the coil matching component (1/4.lambda. for 900 MHZ and 1/2.lambda. for 1800 MHZ). While this antenna has relatively good band width and radiation performances, its length is only about 40 mm. A non uniform helical dual band antenna which is relatively small in size is disclosed in Applicant's copending, commonly assigned application entitled "Multiple Band Non-Uniform Helical Antennas", Ser. No. 08/725,507, now U.S. Pat. No. 6,112,102, the entirety of which is incorporated by reference.
It is known that an extended whip antenna has a higher efficiency when the phone is relatively close to a human head. Typical dual band extendable whip antennas, such as those mentioned above, require a complicated matching network to match the whip antenna impedance to the two bands within 50 ohms. A dual band retractable antenna is disclosed in Applicant's copending, commonly assigned application entitled "Retractable Multi-Band Antennas", Ser. No. 08/725,504,now U.S. Pat. No. 5,963,871 the entirety of which is incorporated by reference. Such an antenna requires two ports, one for a helical antenna and another for a whip antenna. A means for switching between the ports for the different modes is required.